Classically, quintessence was the fifth element, after earth, air, fire, and water – the ephemeral substance that prevented the planets from falling to the center of the celestial sphere. Recently, the term has been used to refer to scalar field models for dark energy. These models may be distinguishable from the cosmological constant as dark energy by future experiments which will probe the effective equation of state (ratio of pressure to energy density) of the dark energy. Unlike the cosmological constant, the energy density in scalar field dark energy models is, in general, time-dependent.